think 1962 I had all but 7 provinces.
That's not very impressing...
The counter to subjugation is making vassals more likely to revolt. If I'm a clan lord and some bully wants to curb-stomp because me he has 150,000 troops and I have 30,000, I'd gladly become his vassal until I get the opportunity to break free, ideally at a moment of over extended weakness. Maybe with a few other ambitious lords. But the AI rarely seems to plot to break free. Maybe they need something like a sub-plot - a conspiracy perhaps, that lets you find out who else is plotting against your lord / neighbors.
I've beaten the game twice now, first as Nagao (quite hard, civil wars saved me twice) and Kono (fairly easy) and I've seen maybe 2-3 AI plots kick in, only 1 involving 2 weaker clans taking on a stronger party. I have tried 1 plot against a stronger party myself, and it was just too much effort arranging marriages and sending bribes. Took me nearly 15 years. In that time, I could have beaten 2-3 smaller clans and had enough clout to stand up to the bigger clan myself.
My point is still this though, why should a 100 province Shogun contender clan go to war against a clan with 3 provinces. There's no system of alliances and guarantees like in the EU games, so the outcome is already known: splat! Unless they were willing to subjugate themselves, they died.
There was a bug with getting external backers to join your plot to break free, so with the patch this will be easier. But even so, in our test games I've seen quite a lot of plotting by the AI, and clans falling apart.